Porous-Nafion/PBI composite membranes and Nafion/PBI blend membranes for vanadium redox flow batteries

Although Nafion membranes have a high chemical stability against VO ₂ ⁺ and a low resistance, their low coulomb efficiency (CE), due to crossover of vanadium cations, should be addressed. PBI membranes are chemically stable and effectively block vanadium cations, but have a lower conductivity than Nafion. Here we describe the fabrication of layered membranes, which consist of a 40 µm thick porous Nafion layer and a 2–17 µm thin PBI blocking layer. To promote adhesion, a <2 µm thick layer of a 1:1 Nafion/PBI blend is introduced between the outer layers. While this bonding layer is necessary to promote adhesion, the strong ionic interactions between Nafion and PBI reduce the acid uptake. Immersed in 1 M sulfuric acid (SA), the weight of meta-PBI increases 17%, while that of NP1:3, NP1:1 and NP3:1 only increases 16%, 8% and 7%, respectively. This decreases the conductivity in 2 M SA from 2.8 mS cm ⁻¹ for meta-PBI to 1.5, 0.4 and 0.04 mS cm ⁻¹ for NP1:3, NP1:1 and NP3:1, respectively. The initial CE of a flow battery using p-Nafion-1:1-PBI20 was as good as one with Nafion 212 (93% at 80 mA cm ⁻² ), while one with p-Nafion-1:3-PBI3 showed exactly the same voltage efficiency as one with Nafion 212.